The present invention relates to magnetic storage apparatus and longitudinal magnetic recording media and more particularly, to a longitudinal magnetic recording medium which is low in media noise and is less susceptible to thermal fluctuation as well as to a magnetic storage apparatus which has a recording density as high as, e.g., 3 giga-bits or more per inch.sup.2.
As computers having higher performances are developed, the quantity of information to be treated such as image data has been steadily increased. This also has required a larger capacity of a magnetic disk device as an external storage device. At present, a recording density of several hundreds of mega-bytes per inch.sup.2 is realized. As a magnetic head for such a high-density magnetic disk device, there has been recently employed an inductive/magneto-resistive composite head which is separated into recording and reproducing parts so that an inductive head is used as the recording part and a magneto-resistive (MR) head is used as the reproducing part. The magneto-resistive head is higher in reproducing sensitivity than the prior art inductive head, so that, even when the size of recorded bits is much reduced and leakage flux is decreased from the medium, a sufficient reproducing or read output can be obtained. Further, there has been developed a spin-valve type giant magneto-resistive head having a much increased reproducing sensitivity.
A longitudinal magnetic recording medium now commercially available is made up of a magnetic layer of a Co alloy such as CoNiCr, CoCrTa or CoCrPt and a Cr underlayer for control of crystallographic orientation of the magnetic layer. Since the Co alloy magnetic layer has a hexagonal close-packed (hcp) structure having a c axis as an axis of easy magnetization, when the magnetic layer is used for a longitudinal magnetic recording medium, it is desirable to align the c axis to be in-plane. To this end, a technique has been employed such that a Cr underlayer having a body-centered cubic (bcc) structure is first formed on a substrate, and a Co alloy magnetic layer is epitaxially grown on the Cr underlayer so that the c axis is oriented to be in-plane. When CoCrPt alloy is employed for the magnetic layer, a technique has been proposed in which Ti or V is added to a Cr underlayer to increase a lattice spacing and to improve a lattice matching between the underlayer and magnetic layer, thus orienting the c axis more closely to be in-plane (refer to JP-A-63-197018, laid open on Aug. 15, 1988 and JP-A-62-257618 laid open on Nov. 10, 1987).
When a MR head is used as a reproducing head, not only a signal but also noise from the longitudinal recording magnetic medium would be picked up with high sensitivity, for which reason the longitudinal magnetic recording medium is required to be lower in noise than the prior art. The media noise results mainly from a region (magnetization transition region) in which magnetization is disturbed between recording bits. Thus narrowing the region leads to reduction in the media noise. As is well known, this is effectively achieved by making fine magnetic crystal grains or magnetic grains of the magnetic layer and weakening interaction between the particles to thereby make magnetization reversal size small. As has been mentioned above, since an epitaxial relationship is established between the magnetic layer and underlayer, magnetic grains can be made fine by making fine particles of the underlayer. Methods for making the underlayer particles fine are considered to include a method for making the thickness of the underlayer small and a method for adding doping elements to the underlayer.
These methods, however, have problems that it is difficult to keep desired crystallographic orientation and to form a film having an excellent crystalline structure, and further it is hard to establish a good epitaxial relationship between the underlayer and the magnetic layer. There occurs another problem of thermally unstable magnetization involved by excessive fine magnetic grains, which results in that a recorded signal decreases with time. An important factor of fabricating a microstructure desirable for high-density recording is to make fine particles and also to make dispersion of particle sizes small to thereby suppress formation of particles which are too fine to be immune to the influence of thermal fluctuation.